American Mineralogist, Volume 100, pages 2484–2496, 2015

Micro- and nano-characterization of Zn-clays in nonsulfide supergene ores of southern Peru

Nicola Mondillo1,*, Fernando Nieto2 and Giuseppina Balassone1

1Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Via Mezzocannone, 8 I-80134 Napoli Italy 2Departamento de Mineralogía y Petrología and IACT, Universidad de Granada, CSIC, Av. Fuentenueva, 18002 Granada, Spain

Abstract Zn-clays are associated with several supergene nonsulfide ore deposits worldwide, where they are either the prevailing economic , or minor components of the weathering-derived assemblage. A TEM-HRTEM study on Zn-clays from nonsulfide ore deposits of Accha and Yanque (Peru) was carried out, to properly determine the chemistry and complex texture of these clays, not fully defined in other previous works on these (but also on other similar) deposits. The Zn-clays oc- curring at Accha and Yanque are constituted by a mixture of and Zn-bearing beidellite. The chemical composition of sauconite varies in a range of values, without any chemical gap, around the average composition:

Ca0.15K0.05(Zn2.1Mg0.2Al0.4Fe0.15Mn0.02)(Si3.5Al0.5)O10(OH)2·nH2O.

Beidellites present an average composition close to stoichiometry with the addition of Zn:

Ca0.05K0.15(Al1.6Zn0.25Mg0.1Fe0.15)(Si3.6Al0.4)O10(OH)2·nH2O.

The chemical composition of both sauconite and beidellite is consistent through the samples, with sauconite affected by a wider variation in composition than beidellite. The textures of Zn-bearing smec- tites clearly indicate that a part of these clays grew on precursory -like phyllosilicates, whereas another part was derived from a direct precipitation from solutions. The occurrence of a paragenesis with trioctahedral and dioctahedral smectites demonstrates that, as observed in other environments, also in a Zn-bearing system both smectite types are stable. As proved for other analogous trioctahedral- dioctahedral smectite systems (e.g., -beidellite), also in the sauconite-beidellite system a chemical compositional gap exists within the series. The texture indicating a direct precipitation from solutions does not exclude that a smectite amount could be genetically related to hydrothermal fluids, even if several other characteristics (e.g., the paragenetical association with Fe-hydroxides typical of gossans) confirm the supergene origin for the bulk of the deposit. Keywords: Sauconite, Zn-beidellite, nonsulfide ore deposits, TEM-HRTEM

Introduction minerals or minor components of the weathering-derived mineral Zn-bearing clay minerals occur in several nonsulfide zinc assemblage (Balassone et al. 2008; Boland et al. 2003; Boni et ores (Hitzman et al. 2003; Large 2001). Zinc nonsulfide deposits al. 2009; Borg et al. 2003; Coppola et al. 2008; Emselle et al. are concentrations of economic Zn-oxidized minerals, mainly 2005; Frondel 1972; Ahn 2010; Kärner 2006). The best example represented by , hydrozincite, , sau- is the world-class Skorpion mineralization (Namibia)—the conite, and , markedly different from sphalerite ores, largest supergene nonsulfide zinc deposit in the world (original typically exploited for zinc (Hitzman et al. 2003; Large 2001). reserves of 24.6 Mt ore at 10.6% Zn)—where sauconite, the Nonsulfide ores are genetically related to supergene or hypogene trioctahedral Zn-bearing smectite (Newman and Brown 1987; processes: the supergene deposits primarily form from the oxida- Ross 1946), predominates over the other Zn-oxidized minerals tion of sulfide-bearing concentrations in a weathering regime, (Borg et al. 2003; Kärner 2006). whereas the hypogene deposits form after mineral precipitation Herein we present the first combined TEM-AEM and HRTEM from hydrothermal or metamorphic fluids (Hitzman et al. 2003). crystal-chemical characterization of natural Zn-clay minerals, Zn-clays are worldwide associated with several supergene associated with two nonsulfide ore deposits in Peru (Yanque and nonsulfide ores, where they are either the prevailing economic Accha). In fact, TEM is pivotal for the characterization of crys- talline materials at nano- and sub-nanometer scale, such as clays (Nieto and Livi 2013), allowing for a wide range of imaging and * E-mail: [email protected] diffraction techniques. When coupled with AEM analytical tools,

0003-004X/15/1112–2484$05.00/DOI: http://dx.doi.org/10.2138/am-2015-5273 2484 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2485

Figure 1. (a) General geologic map of the Andahuaylas-Yauri metallogenic province (modified from Perelló et al. 2003). b( ) Geologic map of the Yanque deposit area (modified from Mondillo et al. 2014a). c( ) Geologic map of the Accha deposit area (modified from Boni et al. 2009).

Table 1. Zn-bearing clay minerals and other phyllosilicates elemental composition and atomic structure down to a single atom Name Ideal formulaa 2+ can be provided as well. The aim of this work is to determine Baileychlore (Zn,Fe ,Al,Mg)6(Si,Al)4O10(OH)8 2+ 2+ Bannisterite KCa(Mn ,Fe ,Zn,Mg)20(Si,Al)32O76(OH)16·4–12H2O chemistry and textures of the Zn-clays occurring in the Yanque (Zn,Al)3(Si,Al)2O5(OH)4 and Accha deposits. These characteristics can be important when 3+ 3+ 2+ Franklinfurnaceite Ca2(Fe ,Al)Mn Mn3 Zn2Si2O10(OH)8 2+ planning a correct metallurgical processing, but also, being strictly Hendricksite K(Zn,Mg,Mn )3(Si3Al)O10(OH)2 Sauconite Na0.3Zn3(Si,Al)4O10(OH)2·4H2O related to environmental conditions during mineral formation, al- a IMA accepted. low to better constrain the genesis of the ore deposits. 2486 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS

Table 2. XRD semi-quantitative analyses of Yanque and Accha samples selected for TEM study, after Boni et al. (2009) and Mondillo et al. (2014) Drillcore Latitude Longitude Drillhole Sample depth from the Sample Hemimorphite Smithsonite Zn-smectite (Y)a (X)a elevation (m.s.l.) top of the core (m) name YA-01 8430548 815103 3562 1.5 YA-A O OO OOO YA-02 8430449 815202 3566 5.0 YA-B OOO – OOO YA-05 8430484 815297 3549 8.5 YA-C OO – OOO YA-13 8430673 815099 3544 9.0 YA-D OO – OOO YA-20 8430461 815295 3553 9.0 YA-E – – OOOO MET1-26 8453672 186758 4287 98.5 ACC – O OOOO a Coordinates: UTM, zone: 18L (Yanque) and 19L (Accha), datum: WGS84; – = not found, O <5 wt%, OO 5–20 wt%, OOO 20–40 wt%, OOOO 40–60 wt%. (Table extends on next page)

An overview on Zn-bearing phyllosilicates Fraipontite is considered a weathering-related , as, A list of Zn-bearing clay minerals and other phyllosilicates is for example, in the Belgian deposits (Coppola et al. 2008), or given in Table 1. Sauconite is the predominant Zn-bearing clay also associated with low-temperature hydrothermal fluids, as in in zinciferous nonsulfide ore deposits (Boni 2005; Hitzman et al. Preguiça mine, Southern Portugal (Will et al. 2014). 2003). It was recognized for the first time in the Uberroth Mine, Baileychlore, the Zn-bearing end-member of the trioctahe- near Friendensville, in the Saucon Valley of Pennsylvania (Genth dral chlorite series, was recognized and validated by Rule and 1875). The validity of the species was later proved by Ross Radke (1988) in a specimen from the Red Dome deposit, North (1946), who also produced the chemical formula still accepted Queensland (Australia). by the International Mineralogical Association (IMA). Sauconite A Zn-phyllosilicate intermediate between chlorite and mica has a saponite-like structure, with a tetrahedral charge related to is the franklinfurnaceite (Peacor et al. 1988), which was solely Al/Si substitutions in tetrahedral sheets (Faust 1951; Ross 1946), recognized in association with willemite in the Franklin mine, while Zn takes the place of Mg in the octahedral positions. New Jersey (U.S.A.). Several experimental studies on the synthesis and stability Up to date, in clearly hydrothermal/metamorphic depos- of sauconite were carried out (Harder 1977; Higashi et al. 2002; its in the U.S.A. (e.g., Franklin, New Jersey) and Australia Kloprogge et al. 1999; Pascua et al. 2010; Petit et al. 2008; Roy (Broken Hill), two types of Zn-mica have been identified, i.e., and Mumpton 1956; Tiller and Pickering 1974). These studies bannisterite (Heaney et al. 1992) and hendricksite (Robert and demonstrated that Zn-smectite can precipitate from solutions of Gaspérin 1985). silicic acid, variously mixed with Zn-compounds (Zn-chloride, Zn-oxide, or Zn-hydroxide), Na-compounds, and Al-compounds, Background information on Peruvian Zn at temperatures ranging between 20 and 200 °C for pH from 6 to clay-bearing deposits 12. The retention of base (Zn) and heavy metals in other phyl- losilicate lattices through adsorption mechanisms, as in , Geological setting has been also investigated (Gu and Evans 2008; Miranda-Trevino The present study is based on the Zn-smectites from the and Coles 2003; Srivastava et al. 2005). Yanque and Accha nonsulfide Zn-Pb deposits, Cuzco region, in Several occurrences of sauconite have been reported world- southern Peru (Boni et al. 2009; Mondillo et al. 2014a). wide, e.g., in the Moresnet-Altenberg nonsulfide deposit (La The Yanque and Accha deposits are located in the Calamine) in Belgium (Coppola et al. 2008; Frondel 1972), in Andahuaylas-Yauri metallogenic province, extending for sev- the supergene weathering zones of the Irish Tynagh and Sil- eral hundred square kilometers around the town of Cuzco. The vermines deposits (Balassone et al. 2008), in the Shaimerden Andahuaylas-Yauri province hosts numerous porphyry copper deposit, Kazakhstan (Boland et al. 2003), in the Sierra Mojada and porphyry-related skarn deposits that are spatially and tem- Zn district in Mexico (Ahn 2010), and in the Reliance deposit porally associated with the middle Eocene to early Oligocene near Beltana, South Australia (Emselle et al. 2005; Hitzman et al. (ca. 48–32 Ma) intrusions of the Andahuaylas-Yauri batholith 2003). In these deposits, sauconite is associated with smithsonite into Mesozoic sediments (Fig. 1a) (Perelló et al. 2003). The and hemimorphite, and is considered a product of the weathering Accha-Yanque Belt covers a wide area located in the middle of of Zn-bearing sulfides. In the Skorpion zinc deposit (Namibia), the Andahuaylas province; it hosts many Zn and Pb ores, as well sauconite mainly occurs as coatings of intergranular spaces and as several porphyry copper deposits of variable sizes. voids; it formed after the breakdown and dissolution of feldspars The Yanque prospect is a Zn-Pb nonsulfide concentration and (Borg et al. 2003; Kärner 2006). located 20 km north of Santo Tomás village. The orebody cov- All the other Zn-bearing phyllosilicates have been gener- ers an approximate surface area of 900 × 500 m, and contains ally considered as rare species in natural Zn-oxidized deposits 26 491 kilotonnes of indicated resources at 2.37% Zn and 2.18% (Hitzman et al. 2003). Pb (1.67% ZnEq cutoff) (Zincore Metals, Inc. 2013). The deposit Fraipontite, a Zn-bearing clay belonging to the kaolinite- consists of several sub-horizontal stratabound bodies that extend serpentine group, was first described by Cesàro (1927), who in depth to more than 100 m. Yanque is hosted by a sedimentary found a “silicate double de zinc et d’aluminum hydrate” in the breccia with lateral facies variations, which stratigraphically Vieille-Montagne Mine (Belgium). The mineral was definitely comprehends parts of the Mara and Ferrobamba Formations validated by Fransolet and Bourguignon (1975), who carried (Pecho and Blanco 1983) (Fig. 1b). The mineralized breccia out a structural characterization of the original specimen and consists of a siliciclastic conglomerate, heteropic to a breccia also proposed the chemical formula actually accepted by IMA. containing dolomite clasts. The emplacement of the original MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2487

Table 2.—Extended Drillcore Chalcophanite Quartz K-Feldspar Calcite Dolomite Kaolinite Goethite

YA-01 – – OO – OO OO OO O O YA-02 – – OO – – – – O OO YA-05 – – OOO O – – – OO – YA-13 – OO OO O – – O – O YA-20 O – OOO O – – – O – MET1-26 OOO – OOO – – – – O – sulfide mineralization produced a phyllic alteration of the (Table 2). These analyses excluded the occurrence in the Yanque sedimentary breccia, characterized by alteration of feldspars and deposit of Zn-bearing phyllosilicates of the (e.g., precipitation of microcrystalline mica (Mondillo et al. 2014a). baileychlore), which have almost the same air-dried pattern of The Accha deposit is a sphalerite mineralization hosted in sauconite, but are characterized by non-expandable characteristics. Mesozoic rocks, almost fully oxidized to smithsonite. The miner- alized zone [6613 kilotonnes of measured and indicated resources Materials at 6.37% Zn and 0.78% Pb (2.20% ZnEq cutoff) (Zincore Metals, For TEM-AEM analyses, we have selected five samples from Inc. 2013)] occupies the hinge of an anticlinal dome that has been the Yanque deposit and one sample from the Accha deposit, exposed by erosion. The nonsulfide concentrations, consisting of by using drillcore sections having medium-high Zn-grade and a mineralized zone 5 to 20 m thick, are continuous along strike moderate/high clay contents, already analyzed by Boni et al. to the west for at least 700 m (Boni et al. 2009). (2009) and Mondillo et al. (2014a). Their mineral assemblages, The main host to mineralization consists of carbonate-clay inferred by semi-quantitative mineralogical X‑ray diffraction, matrix-supported breccias and locally by very thin, quartz-rich are reported in Table 2. conglomerate layers (Fig. 1c). The total thickness of the brecci- Yanque samples are characterized by abundant phyllosili- ated interval, visible both in outcrop and in drill core, varies from cates, in particular sauconite, illite, and kaolinite (Table 2). Sam- 50 to 100 m, whereas individual breccia zones are continuous ples YA-D and YA-E originate from some deeply altered parts over 5 to 20 m downhole (Boni et al. 2009). of the siliciclastic conglomerate, which hosts the Yanque Zn-Pb Both Yanque and Accha nonsulfide deposits formed after mineralization, whereas samples YA-A, YA-B, and YA-C were the oxidation of original sulfide protores, which, together with collected from sandstone-shale lenses within the conglomerate. several Cu-porphyry deposits, are genetically related to the Sample ACC was collected in the Accha deposit from a min- emplacement of the Andahuaylas-Yauri batholith (Boni et al. eralized quartz-rich conglomerate layer with abundant sauconite, 2009; Mondillo et al. 2014a, 2014b). interlayered within the limestone containing the main smithsonite orebody. This sample mostly consists of detrital quartz, Zn-Mn- Mineralogy and petrography of Yanque and Accha ores hydroxides (mostly chalcophanite), and sauconite (Table 2). Sauconite is the most abundant economic Zn mineral in the Yanque deposit (Mondillo et al. 2014a). It was observed in Method association with a Zn-bearing mica (indicated as Zn-illite by The particle morphology and quantitative chemical analyses were obtained the above authors) with Zn in its octahedral site, and with a Zn- using TEM and AEM, respectively. The microscope used was a Philips CM20, bearing kaolinite, where Zn was considered not to be a cation in at the C.I.C. of the University of Granada, operating at 200 kV, with an EDAX the clay structure, but an element adsorbed by the Fe-hydroxides solid-state EDX detector. Lifetime of analyses was 100 s; areas producing dead time higher than 5% were rejected to ensure the thin character required by the associated with clays (Mondillo et al. 2014a). Sauconite in the Cliff and Lorimer (1975) approximation. Analyses were obtained, using STEM Yanque deposit was considered to have been mainly formed mode, from powdered portions deposited on a holey C-coated Au grid. This mode through replacement of K-feldspar and of the host of preparation disperses individual grains of minerals onto the grid surface. Albite, rock by weathering process. Other components of the Yanque , muscovite, spessartine, olivine, titanite, and hemimorphite standards were measured using the same protocol as samples, to obtain K-factors for the transfor- mineralization are hemimorphite, smithsonite, cerussite, and mation of intensity ratios to concentration ratios according to Cliff and Lorimer secondary silver sulfides (i.e., acanthite). The original primary (1975). The structural formulas of smectite and mica were calculated on the basis sulfides are virtually lacking in the deposit. of 22 negative charges, i.e., O10(OH)2. According to the accepted stoichiometry of The Accha nonsulfide mineral association consists mainly smectites (Güven 1988), Fe was considered as bivalent for trioctahedral species of smithsonite and hemimorphite replacing both primary ore (e.g., sauconite), and trivalent for dioctahedral species (e.g., beidellite). The Na content in the Zn-clays was not measured, because of the Na-Zn peaks overlap in minerals and carbonate host rocks. Sauconite is less abundant, the energy-dispersion spectrum. However, as reported by Mondillo et al. (2014a), but it has been detected throughout the deposit with the more ICP-MS analyses on a clay-rich fraction excluded the occurrence of significant abundant smithsonite and hemimorphite. According to Boni et amounts of Na in these minerals, where a maximum content of about 0.5% of this al. (2009) sauconite is genetically related to supergene transfor- element has been detected. Two samples (YA-B and YA-D) were also analyzed in HRTEM mode on thin mation of the potassic alumosilicates, and/or forms the filling of sections, to investigate the microscopic texture of clays. The samples were chosen the remaining porosity of the host rock. considering their different clay association detected at TEM-AEM. Copper rings X‑ray diffraction analyses were carried out on clay separates were attached to representative selected areas of the matrix of thin sections prepared of Yanque samples by Mondillo et al. (2014a) under different with Canada balsam and after ion-thinned, using a Fischione Model 1050 ion mill, conditions. X‑ray diffraction patterns of the air-dried, ethylene and carbon coated. Ion milling was performed at 4 kV and ±10°, until the first hole and ±7° during 20 min for final cleaning. The HRTEM study was performed at the glycol solvated, and heated (550 °C) clay aggregates resulted to CIC of the University of Granada (Spain) using a Titan TEM with XFEG emission be typical of expandable smectites, here identified as sauconite gun, spherical aberration corrector and HAADF detector, working at 300 kV, with 2488 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS a resolution of 0.8 Å in the TEM mode and 2 Å in the STEM mode. EDX spectra packets with lattice fringes whose spacing ranges from 10 to 11 for qualitative identification of minerals and chemical maps were obtained using Å (Fig. 5a), but notable differences have been also revealed. For the Super-X system. example, Figure 5b presents lattice fringes, which show random Results interstratification of smectite and mica with measured spacing of 20 Å. This random smectite/mica interstratification has been rec- Texture of clays ognized by the electron diffraction pattern, where it was possible The texture of the Zn-clays was observed in the samples to measure a non-rational order of very diffuse basal reflections YA-B and YA-D, whose preparation using ion-milling technique with calculated spacing of 11.2 Å, from the (001) spot, 9.8 and 8.9 provided the preservation of the mineral fabric (Figs. 2 and 3). At Å from (002), and 9.8 Å as of (003). Moreover, the EDX spectra low magnification, the two samples present similar characteris- of these areas showed an intermediate composition between those tics, and they generally produce comparable electron diffraction usually found for sauconite and mica. In the same figure (Fig. patterns (SAED). Therefore they will be described together. 5b) a fringe spacing of 18 Å was also measured. Somewhere At a size below 10 μm, smectite forms two types of micro- smectite packets presenting alternating fringes at 13 and 10 Å textures here indicated as “compact clay packages” (CCP) and were detected (Fig. 5c). In a two-dimensional lattice image (Fig. “porous clay aggregates” (PCA) (Figs. 2a and 2b). 5d) of a smectite packet characterized by lattice fringes with CCP (Fig. 2a) are characterized by nearly isoriented clay spacing of 11.6 Å, it is possible to recognize crystallographic packets. This microtexture can have a length up to several coherence from layer to layer and that the 11.6 Å spacing is the micrometers and a thickness below 1 μm, and the packets can sum of 4+7.6 Å. The spacing in the perpendicular direction is be straight or slightly curved. In the packets, the clay layers can 4.5 Å, which corresponds to b/2. Figure 5e presents a smectite be curved and show a wavy microfabric. In the CCP, smectite packet with lattice fringe spacing varying between 16 and 17 grains can overgrow upon mica nuclei, forming a sort of epitaxial Å, and wavy microfabric. In the wavy microfabric it is possible structure (Figs. 2c and 2d). Compact smectite packets produce to see coalescing and lens-like shaped sauconite packets. Other electron diffraction patterns constituted by the superposition of compact sauconite packets with lattice-fringe spacing of 13–15 concentric circles, characteristic of a powder-type diagram, and Å and wavy microtexture, which is reflected in the curved and oblique trends of points, corresponding to various individual lens-like structure of the sauconite layers/packets, can also be crystals partly disoriented between each other (Fig. 4). Both the recognized in the YA-B sample. Smectite layers also exhibit layer powder circles and mono-crystals have a 10 Å spacing, which terminations. Iron hydroxide (goethite) and oxide (hematite) as- indicates the typical collapse of smectitic layers related to the sociated with smectite have a mosaic-domain type texture (Fig. microscope vacuum. Figure 4b shows the electron diffraction 5f), in which the different domains present variable orientation pattern of the smectite packet shown on Figures 2c and 2d, and spacing. epitaxial over a mica grain; both smectite and mica present a It was possible to carry out qualitative chemical analyses 10 Å basal spacing, but they can be easily distinguished by their (EDX spectra) of particles during the observation, with the Titan different crystallinity. Mica electron diffraction pattern shows TEM used for the textural analysis. In this way, together with also some general rows having a spacing of 20 Å, which allows textural information, it was possible to obtain data on crystal the identification of mica 2M polytype. structure and chemistry of phases (STEM-EDX). It was revealed The PCA are characterized by random orientation of the clay that in both CCP and PCA, smectite occurs in the two species, packets, which typically show a very fine grain size, lower than sauconite, and beidellite, identified in the Titan by their qualita- those detected in CCP (Fig. 2b). The random orientation produces tive chemical differences (Fig. 6). In fact, the Zn and Al contents radial to dendritic microtextures and leaves spaces between the of the two kinds of smectites are so different that qualitative grains. The voids of these aggregates are frequently occupied differentiation is straightforward. by Fe-hydroxides (Figs. 2e, 2f, and 3), which can have a spongy texture (Fig. 2e), or also occur as rhombic micro-grains and radial Chemical composition of Zn-clays by TEM-AEM aggregates of oblong crystals (Fig. 2f). Electron diffraction pat- Considering that AEM is not an absolute-composition terns of PCA show that smectite has a turbostratic arrangement. technique, and allows determining only the ratios between the When the porous packets are associated with Fe-hydroxides, various elements, it is usually required to normalize the obtained the electron diffraction pattern shows the superposition of the chemical compositions to the basic formula of the investigated smectite and the Fe-hydroxides patterns. Fe-hydroxides pattern minerals. is compatible with characteristics of goethite. From the analyses of the dispersed mineral grains in the As reported in previous studies (Amouric and Olives 1998; samples, it was ascertained that Yanque smectite is mostly Cuadros et al. 2009), it is difficult to obtain lattice-fringe HR- composed by sauconite; it is associated with a discrete amount TEM images from very hydrated clays, as smectites, because of Zn-bearing beidellite and few grains of illite. Beidellite was of the structural damage caused by the electron beam. Another detected in all the Yanque samples, but only in YA-A, YA-B, and problem is related to the vacuum of the TEM environment and/ YA-E proper contamination-free quantitative chemical composi- or electron irradiation, which cause dehydratation and collapse tions could be obtained. of smectite. It results that the usually measured smectite spacing In the Accha sample, similarly to Yanque, sauconite is the is 10 Å in the case of a complete collapse, or >10 Å in case of most abundant clay mineral. Few beidellite and/or montmorillon- an incomplete collapse. ite grains were also detected, but here they were found intimately At high resolution, the samples generally show smectite associated with sauconite. Consequently, as in some Yanque MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2489

Figure 2. (a) Compact clay packages (Sample YA-B). The gray part is the organic resin used to consolidate the sample before the preparation of the thin section. (b) Porous clay aggregates (Sample YA-B). (c and d) Smectite grains overgrown upon mica nuclei, forming a compact clay package (sample YA-D). By STEM-EDX spectrum, mica contains K as main interlayer cation, and Al as main occupant of the octahedral site. (e) Spongy Fe-hydroxides (white arrow) in a porous clay aggregate (Sample YA-B). (f) Rhombic Fe-hydroxides micro-grains (white arrows) in a porous clay aggregate (Sample YA-D). 2490 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS

Figure 3. (a and b) HAADF image and chemical map of sample YA-B: Fe-hydroxides grew filling pores and covering PCA-type smectites. samples, it was not possible to obtain acceptable contamination- (0.5 < AlIV < 0.8 apfu) higher than Yanque one (0.1 < AlIV < 0.7 free quantitative chemical composition of this beidellite; however apfu) (Fig. 7b). a grain could be analyzed. In both the Accha and Yanque samples, sauconite octahedral In Tables 3 and 4, we report some representative chemical composition remains coherent through the data (Figs. 7 and 8). compositions of Zn-clays from the Yanque and Accha samples, Most of the octahedral site is occupied by Zn, which has been normalized to O10(OH)2. The octahedral sites are occupied by Zn, found to completely fill the site only in one case (3 apfu) (Fig. Mg, Mn, Fe, and Al cations, whereas K and Ca were considered 7c); it generally varies continuously between 2.66 apfu and 1.67 as interlayer cations. apfu, and only in three grains comes down to 1.40 apfu. The Sauconite is characterized by a variable composition (Table octahedral site can be also occupied by Al (0.13–0.72 apfu) (Fig. 3), which could be, in a minor extent, a consequence of the minor 7c), Mg (up to 0.46 apfu, generally lower than 0.30 apfu) (Fig. presence of interstratifications with mica layers as shown in Fig- 7d), Fe (up to 0.57 apfu, generally below 0.40 apfu) (Fig. 7e), ure 5b. can fully occupy the tetrahedral site or decrease and Mn (between 0.04 and 0.48 apfu in a few grains, but gener- continuously up to 3.27 atoms per formula unit (apfu) (Fig. 7a), ally below detection limits). As regards the measured octahedral with the remaining amount compensated by AlIV (Fig. 7b). The Fe, sauconite is often intergrown with Fe-hydroxides and oxides: comparison between Accha and Yanque samples shows that consequently the iron content could be enhanced by the contri- Accha sauconite seems to be characterized by an AlIV amount bution of oxyhydroxides-related Fe in some analytical points.

Figure 4. (a) Electron diffraction pattern of a compact smectite packet (Sample YA-B). (b) Electron diffraction pattern of the smectite packet epitaxial over a mica grain shown in Figures 2c and 2d (Sample YA-D). MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2491

Figure 5. Sample YA-D. (a) Close up view of Figure 2c: sauconite packets with lattice fringes spacing of 10–11 Å. (b) Smectitic packets together with other that shows a random interstratification of smectite and mica with measured spacing of 20 Å and intermediate composition between those of smectite and mica. (c) Smectite packet with alternating fringes at 13 and 10 Å. Sample YA-B. (d) Compact smectite packet showing a two-dimensional fringe spacing of 11.6 and 4.5 Å. (e) Compact smectite packet with lattice fringe spacing variable between 16 and 17 Å and wavy microfabric. (f) Hematite associated with sauconite, showing mosaic-domain type texture. 2492 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS

Figure 6. HAADF image and chemical map (Zn and Al) of sample YA-B: sauconite (green) and beidellite (purple) compact clay packages and porous clay aggregates. The qualitative EDX sauconite spectrum is characterized by a Zn peak more intense than in the beidellite spectrum, instead characterized by a more intense Al peak.

Plotting Zn/Altot vs. Si/Altot a positive correlation is observed, sauconite contains Ca amounts slightly higher than Yanque with ratio values ranging from 1 to 5 for Zn/Altot and from 2 to sauconite, as a result of the charge compensation related to the IV 10 for Si/Altot (Fig. 7f). different Al content (Figs. 7b and 7h). As previously discussed, The interlayer content is represented by K and Ca occurring Na cannot be revealed in the presence of Zn by TEM-AEM, and in variable amounts, within the ranges of 0.00–0.27 apfu and hence has not considered as interlayer cation.

0.00–0.38 apfu for K and Ca, respectively (Figs. 7g and 7h). Correlation of Zn/Altot vs. Ca/K ratios generally show a posi- Calcium is more abundant than K, with the values of the latter tive trend, which is coherent with charge compensation between generally approaching zero. K and Ca are positively correlated the layers (Fig. 7i). (Fig. 7h), and Ca/K ratio ranges between 1 and 8. The Accha As regards beidellite, the following data are associated with

Table 3. Representative structural formulas (apfu) of sauconite from Yanque and Accha deposits YA-A YA-A YA-B YA-C YA-C YA-D YA-D YA-E ACC ACC ACC Si 3.88 3.64 3.64 3.31 3.65 3.49 3.69 3.54 3.38 3.48 3.23 AlIV 0.12 0.36 0.36 0.69 0.35 0.51 0.31 0.46 0.62 0.52 0.77

AlVI 0.36 0.28 0.33 0.72 0.35 0.20 0.34 0.23 0.27 0.48 0.48 Mg 0.11 0.25 0.24 0.26 0.18 0.22 0.05 0.46 0.15 0.16 0.24 Fe2+ 0.13 – 0.15 0.31 0.04 0.18 0.18 0.13 0.20 – 0.09 Zn 1.97 2.34 2.29 1.43 2.18 2.37 2.24 2.08 2.28 2.07 2.02 Mn 0.18 0.00 – 0.02 0.04 – – – ∑ 2.75 2.87 3.01 2.74 2.78 2.97 2.82 2.89 2.90 2.71 2.84

K 0.05 0.05 0.09 0.27 0.07 0.07 0.07 0.05 0.04 0.18 – Ca 0.11 0.14 0.21 0.11 0.18 0.15 0.13 0.20 0.26 0.22 0.31 ∑ charge 0.27 0.34 0.51 0.49 0.43 0.37 0.33 0.46 0.55 0.62 0.62

Note: Calculated on the basis of 12 total anions, O10(OH)2. MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2493

tot Table 4. Representative structural formulas (apfu) of beidellite from of values (Zn/Altot between 0–0.25; Si/Al between 1.27–2.31) the Yanque deposit and no correlations were observed (Fig. 7f). Zn vs. AlVI plot (Fig. YA-A YA-B YA-B YA-E YA-E 7c) shows that the distribution of the two elements is clearly Si 3.51 3.83 3.37 Si 3.62 3.37 AlIV 0.49 0.17 0.63 AlIV 0.38 0.63 related to their charge, with divalent Zn reaching a maximum occupancy of 3 apfu in sauconite, whereas trivalent Al reaches AlVI 1.65 1.49 1.46 AlVI 2.00 2.01 a maximum occupancy of 2 apfu in beidellite. Mg 0.14 0.14 – Mg – Fe3+ 0.17 0.29 0.26 Fe3+ 0.03 0.03 In dioctahedral clay minerals, the interlayer cations are K Zn 0.33 0.14 0.52 Zn 0.03 0.05 and Ca, as already discussed for sauconite, but in beidellite K Mn 0.02 – – Mn – – (max. 0.17 apfu) is prevailing compared to Ca (max. 0.14 apfu) ∑ 2.31 2.06 2.24 ∑ 2.07 2.10 (Fig. 7h). The K and Ca contents are positively correlated, but Na – – – Mg 0.09 0.12 the Ca/K statistic ratio is around 1.2, with absolute ratios ranging K 0.05 0.16 0.14 K 0.02 0.03 Ca – 0.05 0.14 Ca – 0.05 between 0 and 1.5 (Fig. 7h). This correlation, more favorable ∑ charge 0.05 0.26 0.42 ∑ charge 0.19 0.38 to K than Ca is in agreement with the charge compensation 3+ Note: Calculated on the basis of on 12 total anions, O10(OH)2. principle and the higher Al content in beidellite than in sau- conite. Smectites can have some Mg amounts in the interlayer; Yanque samples: beidellite was detected by AEM analysis in few however, no definitive criteria exist on Mg distribution between areas (Table 4), and shows a tetrahedral occupancy very similar the octahedral sheet and the interlayer. Some formulas show- to sauconite, with Si in the range of 3.36–3.83 apfu (Fig. 7a). ing high-octahedral and/or low-interlayer sums could be better The octahedral Al generally varies between 1.29 apfu and the adjusted considering part of Mg as an interlayer cation, which maximum stoichiometric value of 2 apfu (measured only in two could explain such anomalies. Nevertheless, we have accepted grains). In the Zn-bearing beidellites, Zn varies between 0.14 and such possibility only for straightforward cases as those of the 0.54 apfu (Fig. 7c). As in sauconite, the other octahedral cations beidellites in sample YA-E (Table 4). show low values: Mg varies between 0.00–0.31 apfu (Fig. 7d), The other detected clays also show discrete Zn contents. Fe ranges between 0.00–0.38 apfu (Fig. 7e), whereas Mn is Montmorillonite is characterized by an octahedral occupancy lacking, except for 0.02 apfu detected only in one Zn-beidellite almost equally subdivided between Al and Mg-Zn, which have grain. The Zn/Altot and Si/Altot ratios vary for a very short range amounts of 1.06 apfu Al, 0.55 apfu Mg, and 0.39 apfu Zn. An

Figure 7. Chemical compositions of clays from the Yanque and Accha deposits. 2494 MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS

(not deeply investigated in this study). It also evidenced that the

analyses containing Al-Si-H2O-(few Zn) were not corresponding to a Zn-bearing kaolinite, but again to a smectite and specifically to a Zn-bearing beidellite. In our opinion, the rare use of TEM- STEM when studying natural Zn-clays (except the pioneer STEM study of Steinberg et al. 1985), could have hampered a correct identification of other types of Zn-smectite in several ore deposits.

Texture of minerals In the texture here called CCP, smectite frequently overgrows the mica grains. The overgrowing of CCP smectite upon mica nuclei probably indicates that the CCP inherit the crystallographic orientation from previous phyllosilicates, which could act as templates. This is also supported by the presence of the random interstratification of smectite and mica, which has been recognized by the intermediate chemical composition, by lattice fringe images Figure 8. Octahedral chemical compositions of sauconite and where packets showing 20 Å spacing were directly observed, and beidellite on the Zn-AlVI-Fe+Mg diagram. by the electron diffraction pattern. In the latter, it was possible to measure a non-rational order of basal reflections with calculated illite grain with interlayer cationic content of ~0.55 apfu is spacing of 11.2 to 8.9 Å. characterized by 0.04 apfu Zn in the octahedral site. Chemical Direct HRTEM observation of lattice fringes of the compact analysis of micas was not considered to be an objective during sauconite packets often showed a variable spacing, generally AEM data acquisition; hence the micas were not systematically ranging around 10–11 Å, but also varying up to 13, 15, or 18 Å. analyzed. However, several micas completely free of Zn were We interpret these variable thicknesses of the smectite layers as found during the HRTEM analysis (e.g., see Figs. 2c and 2d), possibly related to a different contracting behavior (related to TEM together with others with a minor Zn content. vacuum) of layers, in correspondence to different types of inter- layer cation content (Nieto et al. 1996). An alternative explanation Discussion (especially for the 18 Å spacing) could be an interstratification, This TEM-HRTEM study carried out on natural Zn-smectites however not precisely identified during this study. allowed to obtain new results mainly focused on the very detailed The occurrence in the CCP of coalescing packets, lens-like identification of the clay type, on the mineral texture, and chemi- shaped packets, wavy microtexture, and layer terminations are fea- cal composition. tures similar to those observed in other smectites of various origins (e.g., Mellini et al. 1991; Nieto et al. 1996; Vazquez et al. 2014). Clay type identification The PCA textures are typical of clays directly precipitated from Our results have shown that the Zn-bearing clay fraction of the solutions. Consequently, they are constituted by newly formed Accha and Yanque supergene ores, previously simply identified smectite grown in the cavities existing between the CCP. Spongy or as sauconite (Accha) or sauconite+Zn-bearing illite+kaolinite rhombic goethite and hematite occur in the PCA porosity (Fig. 3). (Yanque), is indeed a mixture of several smectites, i.e., the trioc- This common textural relationship suggests that the Fe-hydroxides tahedral sauconite and the dioctahedral (both Zn-bearing and and oxides are syn- to post-genetic with the precipitations of clays. Zn-poor) beidellite. The occurrence of a Zn-bearing beidellite Like CCP, PCA show an annular electron diffraction pattern seems to have been never ascertained, either in nonsulfide Zn that indicates a turbostratic disorder, typical of most smectites. deposits or worldwide. From a textural point of view, there is no difference between In our opinion, the misinterpretation of the nature of the clay sauconite and beidellite, suggesting a syn-genetic origin. minerals, made in the previous studies is due to the use of protocols Chemical composition of combined XRD and EPMA, routinely applied to the mineralogi- cal evaluation of the ores, on the basis of the previous literature on This first TEM-AEM investigation demonstrates that the this type of deposits. Specifically, bulk-rock XRD analyses allowed chemical composition of sauconite varies in a range of values, to basically identifying the occurrence of smectite in the Accha without any chemical gap, around the average composition: deposit, and of smectite, mica (illite), and kaolinite in the Yanque Ca0.15K0.05(Zn2.1Mg0.2Al0.4Fe0.15Mn0.02)(Si3.5Al0.5)O10(OH)2·nH2O. deposit. When a combination of microbeam analyses as EPMA- EDS was employed, the numerous textural and petrographic As expected, sauconite has been found to have a chemical observations led to automatically consider all the Zn-Al-Si-H2O composition characterized by Zn associated with Mg, Al, Fe, analyses as sauconite, the K-Al-Si-H2O-(few Zn) analyses as Zn- and Mn in the octahedral layer, a variable but significant Al bearing illite, and Al-Si-H2O-(few Zn) analyses as Zn-bearing tetrahedral occupancy, and Ca and K as major interlayer cations. kaolinite. The TEM-STEM-HRTEM technique enabled instead The wide range of measured compositions could partially to test every chemical STEM analysis, by using electron diffrac- be affected also by smectite/mica interstratification, clearly tion and lattice images, and allowed to confirm in the studied observed in the Yanque samples. This can likely influence AlIV, samples the occurrence of sauconite and of a Zn-bearing mica Mg, and interlayer cation contents, and led them to vary more MONDILLO ET AL.: MICRO- AND NANO-CHARACTERIZATION OF Zn-CLAYS 2495 than expected for such type of smectite. second PCA texture, instead, likely opens new genetic scenarios, The only sauconite sample from Accha analyzed here shows because, as reported in literature (Roy and Mumpton 1956; Tiller a composition characterized by an average AlIV content slightly and Pickering 1974; Harder 1977; Kloprogge et al. 1999; Higashi higher than in the above mentioned formula (AlIV ~ 0.65 apfu), et al. 2002; Petit et al. 2008; Pascua et al. 2010), experimental and as consequence of the charges compensation, also by a studies demonstrated that sauconite can precipitate from solu- slightly higher interlayer Ca content (Ca ~ 0.22–0.23 apfu). tions of silicic acid, variously mixed with Zn and other compo- Anyway, it is reasonable to suppose that this chemical feature nents, at temperatures ranging between 20 and 200 °C for pH of related to just one sample could not be fully representative of 6 to 12. Consequently, it is possible to admit that PCA smectite the whole deposit. can also form during hydrothermal processes. Considering the Another remarkable result of this research is that beidellite geological features of the deposits, their strong association with from both these Peruvian nonsulfide deposits is always charac- oxidized sulfides, and the link between PCA and Fe-hydroxides terized by variable but significant Zn contents in its structure. and oxides (typical of gossans and of weathering-related environ- Unfortunately, the analysis of beidellites in the Accha sample ments), we can affirm beyond doubt that most of the smectite is has not produced acceptable results, and only data from Yanque genetically related to supergene processes. However, at least part samples could be presented. of it could have also been precipitated through the hydrothermal Beidellites have an average composition close to stoichiom- fluid circulation, which was active in the area during sulfides etry with the addition of Zn: mineralization or slightly after their deposition, before the for- mation of the Fe-hydroxides and oxides typical of the gossan.

Ca0.05K0.15(Al1.6Zn0.25Mg0.1Fe0.15)(Si3.6Al0.4)O10(OH)2·nH2O. The discovery in the Peruvian nonsulfide Zn deposits of a natural association of smectites belonging to the trioctahedral There are various studies on beidellite containing divalent sauconite and the dioctahedral (both Zn-bearing and Zn-poor) cations, and on its chemical relationship with trivalent smectite beidellite types should be considered not only a simple new end-members, e.g., saponite (Grauby et al. 1993, and references mineral finding, but also an important methodological clue for therein). They showed that the natural trioctahedral-dioctahedral future mineralogical evaluation of Zn-nonsulfide deposits with smectite series is discontinuous with large chemical gaps. From possible processing implications. our data, a chemical gap also exists between sauconite and Zn- bearing beidellite, with an effective maximum Al content in Acknowledgments N. Mondillo thanks M. Boni for her constant support during this research. This sauconite around 0.50 apfu and a minimum content in beidellite work was partially financed by Research Projects CGL2011-30153-C02-01 and around 1.30 apfu. CGL2012-32169 (Spanish Ministry of Science) and the Research Group RNM-0179 of the Junta de Andalucía, and supported by the Università degli Studi di Napoli The Zn/Altot vs. Si/Altot positive correlation registered for “Federico II” grant RDIP2013 to G. Balassone. sauconite, against the very short range of values of the same ratios of beidellites, demonstrates that the relative sauconite References cited composition is more variable than beidellite composition in the Abad, I., Jimenez-Millan, J., Molina, J.M., Nieto, F., and Vera, J.A. (2003) Anoma- studied samples. 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